This invention relates generally to radiation therapy equipment for the treatment of tumors or the like and, in particular, to a method of aligning a patient with a previously prepared radiation treatment plan.
In external source radiation therapy, a radiation source external to the patient projects a beam of radiation toward a treatment region. The radiation beam may be x-rays or electrons from a linear accelerator in the range of two to twenty-five MeV or gamma rays from highly focused radioisotopes such as 60Co.
Typically, the tumor will be treated from a number of different angles with the intensity and/or shape of the radiation beam adjusted for each angle. Multiple beams converging on the site of a tumor reduce the dose of radiation received by surrounding non-tumorous tissue and skin and the adjustment of the intensity of radiation at each angle allows irregular tumors to be treated while minimizing the irradiation of sensitive structures near the tumor site. The angles and intensities of the radiation beams for treating a tumor together form a “treatment plan”.
One highly accurate technique of radiation therapy uses a radiation source that produces a fan beam in which the spatial distribution of the intensities may be controlled. The radiation source orbits so that the fan beam illuminates a slice of the patient while the intensity of each small region of the fan beam is modulated as a function of that angle. By properly selecting the beam intensities at different angles, complex regions within the slice or slices may be accurately irradiated. U.S. Pat. No. 5,317,616 issued May 31, 1994, and assigned to the same assignee as the present application, describes the construction of such machines and methods of calculating the necessary beam intensities as a function of angle. Here the treatment plan is a “sinogram” of intensities as a function of angle.
In order to take advantage of the improved accuracy offered by such radiation therapy systems, the treatment plan may be developed using x-ray images of the patient and tumor, either a set of computed tomography (“CT”) images or conventional x-ray images taken at different angles about the patient.
A physician may outline the tumorous area on the x-ray images and then use a computer program that selects the beam angles based on desired dose limits for the tumor region.
The patient may not have the same position on a machine used to acquire images used in treatment planning as they have on the radiation therapy machine. Shifts in patient position are inevitable in treatments that require multiple sessions separated in time and even when the same machine can be used to image the patient and provide radiation therapy, the patient may move slightly in the time it takes to complete the treatment planning.
To address the problem of patient movement, U.S. Pat. No. 5,673,300, also assigned to the assignee of the present invention, describes a method of determining movement of the patient by obtaining one or more x-ray images at the time of treatment and comparing them to the planning images. The treatment plan may then be modified according to the known displacement of the patient or the patient repositioned.
Particularly for sophisticated treatment plans describing many individually modulated radiation beams, changes of the treatment plan to accommodate patient movement can be difficult. On the other hand, repositioning of the patient on the treatment table is also a problem both because it is difficult to gauge how far the patient must be moved (without multiple verification images) and because it may encourage additional movement and/or be difficult or uncomfortable for the patient.
Some movements of the patient, for example, along the axis of movement of the table or in elevation may be corrected mechanically by changing the elevation of the table or its axial position. Changes in angle of the patient with respect to the table axis, for example, in roll may be corrected simply by offsetting the angle of the treatment plan.
On the other hand, angular motion of the patient about a vertical axis (yaw) or a horizontal axis (pitch) cannot be so easily compensated with standard tables. Adding pitch or yaw motions to a table would be prohibitively expensive and by angling the table out of axis increases the possibility of interference between the table and the radiation therapy machine. Any motion that tips the patient from horizontal can cause undesirable motion of internal organs and possibly patient discomfort.